The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is therefore an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA framework type, such as SAPO-34.
Regular crystalline molecular sieves, such as the CHA framework-type materials, are built from structurally invariant building units, called Periodic Building Units, and are periodically ordered in three dimensions. Disordered structures showing periodic ordering in less than three dimensions are, however, also known. One such disordered structure is a disordered planar intergrowth in which the building units from more than one framework type are present. Such intergrowths frequently have significantly different catalytic properties from their end members.
For example, the zeolite ZSM-34 is a well known intergrowth of ERI and OFF framework-type molecular sieves and exhibits an MTO performance far superior to its individual component materials. More recently, silicoaluminophosphate molecular sieves comprising at least one intergrown phase of an AEI framework-type material and a CHA framework-type material have been synthesized and have been found to be particularly attractive catalysts for oxygenate-to-olefin reactions.
For example, U.S. Pat. No. 6,334,994 discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is said to be an AEI/CHA mixed-phase composition. In particular, RUW-19 is reported as having peaks characteristic of both AEI and CHA framework-type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8. RUW-19 is reported to be active as a catalyst in the production of light olefins from methanol (MTO).
In addition, International Patent Publication No. WO 02/70407, published Sep. 12, 2002, discloses a silicoaluminophosphate molecular sieve, now designated EMM-2, comprising at least one intergrown form of molecular sieves having AEI and CHA framework types, wherein said intergrown form has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve. EMM-2 has been found to exhibit significant activity and selectivity as a catalyst for the production of light olefins from methanol (MTO).
The Periodic Building Unit for both AEI and CHA framework-type molecular sieves is the double six-ring layer. A number of other molecular sieves are known to have the same or similar double six-ring layer as their building unit, including AFX framework-type materials. According to the present invention, a new intergrown material of the AFX and CHA framework-type molecular sieves has been synthesized and has been found to exhibit activity as a catalyst in the conversion of oxygenates to olefins.
U.S. Pat. No. 5,370,851 describes the synthesis of SAPO-56, a silicoaluminophosphate of the AFX framework type, using N,N,N′N′-tetramethylhexane-1,6-diamine as a directing agent. According to the '851 patent, SAPO-56 is useful as a catalyst in a large variety of hydrocarbon conversion processes, such as cracking, hydrocracking, alkylation of both aromatics and isoparaffins, isomerization, polymerization, reforming, hydrogenation, dehydrogenation, transalkylation, dealkylation, hydration, dehydration, hydrotreating, hydrodenitrogenation, hydrodesulfurization, methanation and the syngas shift process.
U.S. Published Patent Application No. 2004/0253163, published Dec. 16, 2004, discloses the synthesis of a silicoaluminophosphate molecular sieve having the CHA framework type employing a directing agent of the formula:R1R2N—R3 wherein R1 and R2 are independently selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms and hydroxyalkyl groups having from 1 to 3 carbon atoms and R3 is selected from the group consisting of 4- to 8-membered cycloalkyl groups, optionally, substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms; and 4- to 8-membered heterocyclic groups having from 1 to 3 heteroatoms, said heterocyclic groups being, optionally, substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms and the heteroatoms in said heterocyclic groups being selected from the group consisting of O, N, and S. Preferably, the directing agent is selected from N,N-dimethylcyclohexylamine, N,N-dimethylmethyl-cyclohexylamine, N,N-dimethylcyclopentylamine, N,N-dimethylmethyl-cyclopentylamine, N,N-dimethylcycloheptylamine, N,N-dimethylmethylcycloheptylamine, and most preferably is N,N-dimethylcyclohexylamine.